FIG. 1 illustrates a sectional view of a known isolation assembly 10. The assembly 10 includes upper and lower bushings 12 and 14, respectively, and upper and lower rubber grommets 16 and 18, respectively. The bushings 12 and 14 and grommets 16 and 18 are separate and distinct from one another. That is, when the assembly 10 is not compressively secured between a fastener and a work-piece, the grommets 16 and 18 would simply detach from the bushings 12 and 14.
Each bushing 12 and 14 includes a hollow cylindrical shaft 20 integrally formed with an outturned, planar collar 22 that is generally perpendicular to the shaft 20. Each grommet 16 and 18 includes a generally circular main body 24 defining a central opening. The main bodies 24 fit around the shafts 20.
As shown in FIG. 1, the isolation assembly 10 is used to isolate a fastener 26 and/or work-piece (not shown), such as a bolt, from a work-piece 28, such as a frame, panel, or the like. The work-piece 28 includes a through-hole 30 into which a shaft 32 of the fastener 26 passes. Additionally, the shafts 20 of the bushings 12 and 14 pass into the through-hole 30, as well as portions of the grommets 16 and 18. In this manner, the fastener 26 and the bushings 12 and 14 are isolated from the work-piece 28. Only the rubber grommets 16 and 18 contact the work-piece 28, thereby protecting the work-piece 28 form scratches, gouges, and the like.
The grommets 16 and 18 are compressively sandwiched between the collars 22 of the bushings 12 and 14 and the work-piece 28. As the rubber of the grommets 16 and 18 sets from standard loading, the amount of force exerted into the grommets 16 and 18 (that is, the “load”) is directly affected. Over time, the force exerted into the grommets 16 and 18 may weaken the grommets 16 and 18. Moreover, the rubber within the grommets 16 and 18 may degrade and become weak.
A typical rubber grommet, such as the grommets 16 or 18, is compressed by the fastener head 34 in conjunction with the collar 22 of the bushing 12, for example. As the grommets 16 and 18 permanently set, the effective amount of compression experienced by the grommets 16 and 18 is reduced. Consequently, the grommets 16 and 18 provide less protecting and isolating load to the work-piece 28.
In general, an increase in load exerted into the work-piece 28 may result in substantial movement, such as shifting, of the work-piece 28 itself and/or the fastener 26. Consequently, the grommets 16 and 18 may permanently set in uneven positions, thereby further diminishing proper isolation. In general, over time, a rubber grommet 16 or 18 in compression will permanently set. Typically, permanent setting of the rubber results in a height change and diminishes load and isolation benefits.
During peak operating conditions, a force increase of 500-1000% may be exerted into the grommets 16 and 18. During this time, the grommets 16 and 18 typically allow too much movement for the system to continue to function properly.